Add support for jsr registry and jsr: protocol for the imports.

[JayaKrishnaNamburu]

When installing some packages from denoland. Few of the packages are using jsr: registry and the protocol for its imports. This is an issue to keep track for the progress of jsr support in @jspm/generator

import { Generator } from "@jspm/generator";

const generator = new Generator({
  defaultProvider: "denoland",
  defaultRegistry: "npm",
});
await generator.install("denoland:oak/body.ts");
console.log(generator.getMap());

Sandbox to reproduce the same

Once the implementation is done, the tests need to be re-enabled in @jspm/generator and jspm-cli respectively.

[guybedford]

Yes, these would be great to support! I have some prototype work, sounds like a good next step after we complete the release and update for the integrity story.

[guybedford]

WIP at #372.